The measure, proposed by Assemblyman Mark Leno, would have defined marriage as a union between two people, rather than between a man and a woman. Leno, a San Francisco Democrat, proposed a similar bill in 2005. It was also vetoed.

The Republican governor has pledged to veto all such bills. He said voters and the state Supreme Court should decide the issue. The California Supreme Court is likely to rule next year on whether the state's voter-approved ban on homosexual marriage violates the constitution.

In his veto message Governor Schwarzenegger voiced support for domestic partnerships that provide many of the rights and responsibilities of marriage.